With continuous development and advancement in mobile telecommunications technologies, mobile telecommunications networks in operation providing mobile telecommunications services tend to include a mixture of legacy equipment and new equipment that implement legacy standards/protocols and new standards/protocols, respectively. For example, a mobile telecommunications network operated by a given service provider at present day typically includes equipment that implement legacy standards and protocols, such as 2nd Generation (2G), e.g., Global System for Mobile Communications (GSM), and/or 3rd Generation (3G), e.g., Universal Mobile Telecommunications System (UMTS), standards and protocols, as well as equipment that implement the latest standards and protocols, such as 4th Generation (4G) including Long Term Evolution (LTE) and/or Mobile WiMax, standards and protocols. That is, a present-day mobile telecommunications network is likely to include one or more 4G networks, one or more 3G networks and/or one or more 2G networks.
In a 4G network, System Architecture Evolution (SAE) is the core network architecture of LTE wireless communication standard, and data communication of 3rd Generation Partnership Project (3GPP) in SAE is packet-based. It does not have a circuit-switched domain, which is traditionally used for phone calls and Short Message Service (SMS). Circuit-switched fallback (CSFB) is a feature specified in 3GPP that allows a mobile user device (referred to as user equipment (UE) in the context of 4G/3G and mobile subscriber (MS) in the context of 2G) to change its radio access technology from LTE to a 2G/3G technology which supports circuit-switched services, in order for the mobile user device to make and receive calls.
However, when the connectivity between a pooled 3G radio network controller (RNC) (or 2G base station center (BSC)) and a pooled mobile switching center (MSC) is lost, all LTE subscribers (i.e., users of affected mobile user devices) served by the affected RNC/BSC will not be able to receive calls until the connectivity is re-established. As a result, mobile terminating calls to LTE subscribers are lost and the LTE subscribers end up missing the calls. The 3GPP standards do not cover this situation and the mobile terminating calls to the LTE subscribers served by the RNC/BSC suffering a failed luCS interface will fail and get redirected to the recipient's voice mail.